The present disclosure generally relates to a welding method for multi-layer polymeric or plastic strips. The welding method is particularly useful for geosynthetic products and especially a polymeric cellular confinement system for reinforcing geotechnical materials.
Polymeric or plastic soil reinforcing articles, especially cellular confinement systems (CCSs), are used to increase the load bearing capacity, stability and erosion resistance of geotechnical materials such as soil, rock, sand, stone, peat, clay, concrete, aggregate and earth materials which are supported by said CCSs.
CCSs comprise a plurality of high density polyethylene (HDPE) or medium density polyethylene (MDPE) strips in a characteristic honeycomb-like three-dimensional structure. The strips are attached or welded to each other at discrete locations to achieve this structure. Geotechnical materials can be reinforced and stabilized within or by CCSs. The geotechnical material that is stabilized and reinforced by the said CCS is referred to hereinafter as geotechnical reinforced material (GRM). The surfaces of the CCS are sometimes embossed to increase friction with the GRM and decrease relative movement between the CCS and the GRM.
The term “HDPE” refers hereinafter to a polyethylene characterized by density of greater than 0.940 g/cm3. The term medium density polyethylene (MDPE) refers to a polyethylene characterized by density of greater than 0.925 g/cm3 to 0.940 g/cm3. The term linear low density polyethylene (LLDPE) refers to a polyethylene characterized by density of 0.91 to 0.925 g/cm3.
The polymeric walls of the CCSs may become damaged during service and use in the field by UV light, heat, and humidity (collectively, UHH). The damage results in brittleness, decreased flexibility, toughness, impact and puncture resistance, poor tear resistance, and discoloration. In particular, heat damage to the CCS is significant in hot areas on the globe. As used herein, the term “hot areas” generally refers to areas located 42 degrees latitude on either side of the equator and especially along the desert belt. Hot areas include, for example, North Africa, southern Spain, the Middle East, Arizona, Texas, Louisiana, Florida, Central America, Brazil, most of India, southern China, Australia, and part of Japan. Hot areas are characterized by a combination of temperatures as high as +90° C. on dark surfaces exposed to direct sunlight, and intensive sunlight for periods of up to 14 hours each day.
Some strategies have been applied industrially in order to protect the plastic walls from this damage by treating the polymer making up the plastic walls. For dark colored products, e.g., black or dark gray products, carbon black can be introduced to block UV light and dissipate free radicals. However, one disadvantage produced through the use of carbon black is its aesthetic appearance. Black CCSs are less attractive in applications where the CCS is part of a landscape structure. A second disadvantage is that black CCSs tend to absorb sunlight and heat up. As a consequence, creep can be severely accelerated, especially in the welding points and in thinner wall structures, potentially resulting in structural failures.
CCSs are usually immobilized relative to the GRM by wedges, tendons, or anchors. This immobilization is especially crucial when the CCS is used to reinforce a slope. These anchor points are subjected to severe stress concentrations. Without UHH protection, these anchor points may fail before any significant damage is observed in the rest of the CCS.
Stress is also generated at the welds between the strips making up the CCS. Stress can be applied from compression when trucks drive over the CCS before it is filled with GRM or when GRM is dumped onto the CCS to fill the cells. GRM can also expand when it becomes wet or when water already in the GRM freezes in cold weather. In addition, GRM has a coefficient of thermal expansion (CTE) about 5-10 times lower than the HDPE used to make the strips. Thus, the HDPE will expand much more than the GRM; this causes stress at the welds as well.
Some CCSs are pigmented to shades similar to the GRM they support. These include light colored products and custom-shaded CCSs, such as soil-like colored CCSs, grass-like colored CCSs and peat-like colored CCSs. For these CCSs, special additives (i.e. other than carbon black) are required in order to maintain their properties for periods as long as 20 years. The most effective additives are UV absorbers such as benzotriazoles and benzophenones, radical scavengers such as hindered amine light stabilizers (HALS), and antioxidants. Usually, “packages” of more than one additive are provided to the polymer. The additives are introduced into the polymer, usually as a master batch or holkobatch, a dispersion, and/or solution of the additives in a polymer carrier or a wax carrier.
The amount of additives in the polymer used to make the CCS depends on the life-time required for the CCS. To provide protection for periods of about 5 years, the amount of additives needed is less than if protection for a period of 10 years or more is required. Because additives leach out of the polymer, evaporate, or hydrolyze over time, the actual amount of additives required for protection over a long period of time is about 2 to 10 times greater than the amount that is needed for short term protection needs. In other words, the amount of additives added to the polymer is not a linear function of the time for which protection is desired.
The additives are generally dispersed or otherwise dissolved fairly evenly throughout the entire cross-section of the polymeric strips used to make the CCS. However, most interaction between the additives and the UHH damage-causing agents takes place in the outermost volume, i.e. 10 to 200 microns, of the polymeric strip or film.
U.S. Pat. No. 6,953,828 discloses a membrane, including a geomembrane, stabilized against UV. The patent relates to polypropylene and very low density polyethylene compositions that are effective as membranes, but are not practical for CCSs. Polypropylene is too brittle at sub-zero temperatures. Very low density polyethylene is too weak for use in a CCS because it tends to creep under moderate loads. Once a CCS creeps, the integrity of the CCS and GRM is disrupted and structural performance is irreversibly damaged. In addition, polypropylene requires a large loading of additives to overcome leaching and hydrolysis; this results in an uneconomical polymer.
U.S. Pat. No. 6,872,460 teaches a bi-layer polyester film structure, wherein UV absorbers and stabilizers are introduced into one or two layers. Various grades of polyesters are generally applicable for geo-grids, which are two-dimensional articles used to reinforce soil, such as a matrix of reinforcing tendons. In contrast, CCSs are three-dimensional. Polyesters are generally unsuitable for CCSs due to their stiffness, poor impact and puncture resistance at ambient and especially at sub-zero temperatures, medium to poor hydrolytic resistance (especially when in direct contact with basic media such as concrete and calcined soils), and their overall cost. Again, polyesters require a large loading of additives to overcome leaching and hydrolysis; this results in an uneconomical polymer.
For thin polymeric strips (characterized by a thickness of less than about 500 microns), the amount of additive required generally matches the theoretical calculated required amount. In thicker strips (characterized by thickness of more than about 750 microns—that is usually the case with structural geotechnical reinforcing elements—CCS as example), however, the total amount of additive required is generally higher than the theoretical calculated required amount. For high performance CCSs having thicknesses of about 1.5 mm or more, wherein strength, toughness, flexibility, tear, puncture resistance, and low temperature retention are required, the total amount of additive required is generally 5 to 10 times higher than the theoretical calculated required amount. UHH-protecting additives are very expensive relative to the cost of the polymer. Most manufacturers therefore provide the additives at loadings more closely matching the low (i.e. minimal) theoretical calculated loading level, not the higher loadings required for long-term protection for periods of 50 years and more. Because of this, most manufacturers do not currently guarantee long-term durability of their thick polymeric strips. Current CCSs use HALS and UV absorbers in the amount of 0.1 to 0.25 weight percent dispersed throughout the polymeric strip.
Another aspect related to outdoor durability is the type of polymer used for the CCS. Selection of the correct polymer for this application is a tradeoff between economy, i.e. cost of raw materials, and long-term durability. In this regard, polyethylene (PE) is one of the most popular materials for use, due its balance of cost, strength, flexibility at temperatures as low as minus 60° C., and ease of processing in standard extrusion equipment. Moreover, polyethylene is moderately resistant against UV light and heat. However, without additives, polyethylene is susceptible to degradation within one year to a degree that is unacceptable for commercial use. Even when heavily stabilized, PE is still inferior relatively to more resistant polymers—ethylene-acrylic ester copolymers and terpolymers for example.
On the other hand, polymers that exhibit higher UV and heat resistance, such as acrylic and methacrylic ester copolymers and terpolymers, and specifically ethylene-acrylic ester copolymers and terpolymers, are very suitable to commercial application from the standpoint of UHH resistance. However, their relatively high cost and relatively low modulus and strength characteristics limit their wide-scale use in CCS applications.
A preferred and cost effective method for joining the strips to a CCS, is ultrasonic welding. Ultrasonic welding is suitable for most thermoplastic materials, and is widely used in the automotive, packaging, electronic, and consumer industries. An ultrasonic welding system typically contains a high-frequency power supply (usually 20-40 kHz). The high-frequency energy is directed into a horn (also known as a sonotrode), which is a bar or a metal section dimensioned to be resonant at the applied frequency. The horn contacts the surface of or penetrates into the plastic material which is to be welded and transmits mechanical vibrations into it.
Typically, it is desired to join to plastic parts together. The plastic material should have some means of alignment and a small, uniform initial contact area at the desired joint or interface to concentrate the ultrasonic energy for rapid localized energy dissipation. An energy director, the most commonly used design, consists of a small triangular bead of material at the desired joint or interface area. A combination of applied force, surface friction, and intermolecular friction increases the temperature of the plastic parts until the melting point is reached. The interfaces melt and telescope together, producing a weld in the shear mode. The ultrasonic energy is then removed, leaving a molecular bond or weld between the two plastic parts.
Ultrasonic welding is more efficient in relatively rigid materials and relatively amorphous ones. Usually, high welding frequency is related to low melting rate and lower pressure, as well as more shallow penetration. Ultrasonic welding is very difficult in thin films and is usually applied only to films having a thickness greater than 0.5 mm. Ultrasonic welding is also very difficult relatively soft and low specific gravity polymers, such as polyethylene, that are common materials in geosynthetics, including CCS.
In a simple, monolithic, one-layer, thick-strip based CCS, the welding is provided by ultrasonic means, usually in the range of 15-20 MHz. For example, a method of assembly is described in Russian Patent Nos. 2,152,479 and 2,152,480, wherein pressure and heat are provided to form a joint.
In single-layer strips, it is generally desired to evenly weld the strip throughout its entire cross-section. However, the situation is different in a multi-layer strip. In a multi-layer strip, the welding should be focused in the outer layers for optimal strength and minimal damage to UV protection in the weld area.
Some references provide technology for thin layers ultrasonic welding. U.S. Pat. No. 5,411,616 provides a method for ultrasonic welding of thin plastic films. The method is applicable for an engineering thermoplastic such as polycarbonate, but not for softer plastics such as polyethylene, the most common material in CCS.
It would be desirable to be able to weld a multi-layer plastic strip using ultrasonic means, wherein the welding energy is applied mostly in the outer layer(s) of the strip and does not affect the UV protection of the outer layers.